Modulation of cellular signaling mechanisms by qualitative differences in dietary proteins and their metabolites were studied at the level of: a) Imidodipeptides, b) Pyrroline 5-carboxylate reductase as a mediator of redox exchange, and c) Effect of pyrroline 5-carboxylate on mitogenesis-effect on membrane phosphoinositides a. Imidodipeptides. Dipeptides containing proline or hydroxyproline originate from either tissue matrix degradation or from protein nutrition. They circulate in plasma and are delivered to tissues where they are hydrolyzed by prolidase. Thus, prolidase is a potential interface between protein nutrition and matrix breakdown. Our studies showed that the level of cellular prolidase is regulated by extracellular collagen acting through integrin receptors. Thus, the hydrolysis of imidodipeptides, the final degradative products of matrix collagen, is responsive to cellular interaction with extracellular matrix. We are studying the regulation of this enzyme on the molecular level. b. Pyrroline 5-carboxylate reductase. We are studying this enzyme, which catalyzes the committed step in proline biosynthesis, on the molecular level. Previous studies suggested that it also functions in plasma membrane redox transfers. Using Western blots to analyze cellular fractions, we have shown that the enzyme is associated physically with cellular plasma membranes. The molecular mechanisms for this association is being investigated. c. Effect of pyrroline 5-carboxylate on mitogenesis. P5C stimulates PRPP and purine ribonucleotide synthesis synergistically with platelet-derived growth factor. It also increases the incorporation of thymidine in serum- activated cells. Inhibitor studies suggest that the effect is due to the turnover of membrane phosphoinositides, and direct assays show that it is phosholipase D which is stimulated by P5C with the release of phosphatidic acid. Possible interaction of this system with various growth factors and its modulation of intermediate metabolism are being pursued.